The present invention relates to superconductors and more particularly to Type II superconductors made of intermetallic compounds of Beta-Wolfram structure.
Circa 1955, Matthias and co-workers at Bell Labs announced the discovery of niobium stannide Nb.sub.3 Sn and its 18.degree.K critical temperature which offered the promise of high critical fields and high critical currents. In the 1960's other Beta-Wolfram compounds were announced including V.sub.3 Ga and many ternary and quatenary compounds based on Nb.sub.3 Sn and V.sub.3 Ga which afforded critical temperatures of 15.degree. to 20.degree.K and above. Great efforts were expended to make practical fabricable and usable forms of these compounds. In 1960-1963, Bell Laboratories, Superior Tube Corporation and Materials Research Corporation in the United States developed Nb.sub.3 Sn wires made by packing niobium and tin powders or crushed Nb.sub.3 Sn into niobium, Monel or copper tubes, drawing the packed tubes to wire size and diffusion heating to form superconductive cores of Nb.sub.3 Sn. At about the same time, General Electric Company developed a tin coated niobium wire which was similarly heated to produce an Nb.sub.3 Sn layer, National Research Corporation developed co-reduced, metallurgically bonded, niobium-tin laminates which were similarly heated to produce Nb.sub.3 Sn layers, Radio Corporation of America developed a chemcial vapor deposited Nb.sub. 3 Sn coating method, and Union Carbide developed an Nb.sub.3 Sn plasma spraying process. Similar developments were made at other major U.S. industrial, government and university laboratories and abroad and offered commercially in the U.S. and abroad. Follow-up developments included soldering on stabilizing copper layers and the use of bronze sources of the tin or gallium components of the Beta-Wolfram compounds.
However, use of the Beta-Wolfram compounds was small compared to the lower critical current alloys Nb-Zr, Nb-ti, and Nb-Zr-Ti, because of greater stability, ease of fabrication and ductility of these alloys. A typical composite braided or cabled wire product based on the alloys is specified to carry 1000 amperes superconductively under 40 kilogauss external field and each wire of the braid comprises about 400 spaced niobium-titanium filaments in a stabilizing pure copper matrix.
Work in the U.K. (see U.S. Pat. No 3,728,165 to Howlett and U.S. Pat. No. 3,472,944 to Morton et al. and U.S. Pat. No. 3,807,04l to McDougall), in the U.S. (see U.S. Pat. 3,731,374 to Suenaga et al. and U.S. Pat. No. 3,838,503 to Suenaga et al.) and in Japan (see Applied Physics Letters January, 1974, Furuto et al.) has involved the production of Type II superconductors by reaction of niobium or vanadium with bronze containing gallium or tin. Such products are produced as wires comprising spaced filaments of niobium (for example) in a tin bronze matrix which has been drawn to a small cross sectional area. After drawing, the surface of the niobium is converted to Nb.sub.3 Sn by a high temperature diffusion of tin from the bronze matrix into the niobium. While materials of the above type show considerable promise for high field superconductors they are somewhat limited in their stability and ultimate current carrying capacity.
It is an important object of the invention to provide Beta-Wolfram compound based wire and wire braid or cable products having stability approaching that of the above alloy products.
It is a further object of the invention to provide such Beta-Wolfram compound based products which are sufficiently ductile to be used in electromagnetic coil winding and like wire or wire braid and cable fabrication and usage.
It is a further object of the invention to produce such Beta-Wolfram compound based products with 2 hours or less of diffusion heating consistent with one or more of the preceding objects.
It is a further object of the invention to provide diffusion heating in a continuous tunnel kiln to eliminate spooled wire batch heat treatments.
It is a further object of the invention to make available the practical utilization of the high field, high current characteristics of the Beta-Wolfram compounds together with the electrical stability and mechanical characteristics of alloy conductors for magnetic coils, rotating electrical machinery, power transmission and the like.
As used herein, "wire" includes wires and wire-like ribbons, flattened wires and the like. Resistance ratio means ratio of electrical resistance measured at room temperature (300.degree.K) to resistance measured at liquid helium temperature (4.2.degree.K).